Neutralization devices are currently available for use in neutralizing charged particles for a variety of applications, such as, neutralization of charged nanometer particles for use in the development of standards, the use of such neutralized particles for structured materials, the use of neutralized particles for biotechnology applications, etc. For example, a neutralization device is described in U.S. Pat. No. 5,247,842 (Kaufman et al.), entitled, "Electrospray Apparatus For Producing Uniform Submicrometer Droplets," issued Sep. 28, 1993. The neutralization device is used in combination with an electrospray device. In the electrospray device, electrically conductive liquid is supplied at a controlled rate to a capillary tube. A voltage differential between the capillary tube and a surrounding chamber wall creates an electrostatic field that induces a surface charge in the liquid emerging from the tube. Electrostatic forces disperse the liquid into a fine spray of charged droplets. To produce the spray, each droplet is charged to about 80-95% of the Rayleigh limit (at which point electrostatic repulsion overcomes surface tension). Such electrospray devices are used in many applications due to their ability to generate small and uniform droplets.
The electrically conductive liquid being sprayed is generally a liquid having particles dispersed therein. The particles, e.g., particles of a suspension, and the liquid is sprayed using the electrospray device to form a spray of small droplets. The droplets are then dried, and the particles are left in aerosol form. The particles, may then be, for example, studied or analyzed using downstream analysis devices, e.g., detectors and apparatus, such as differential mobility particle sizers (DMPS), differential mobility analyzers (DMA), electrometers, and condensation particle counters (CPC). The charged particles resulting from use of the electrospray device may have, for example, a nominal diameter of about 100 micrometers or less.
As liquid evaporates from the droplets, surface charge density on the droplets increases until the Rayleigh limit is reached, at which point the coulomb repulsive force becomes on the same order as cohesive forces, such as surface tension. The resulting instability causes the original droplet, sometimes referred to as the parent or primary droplet, to disintegrate into smaller droplets, thus, the resulting distribution of droplet size is broad, i.e., nonuniform. One solution to the problem is to neutralize the droplets and, as such, the particles.
As described in U.S. Pat. No. 5,247,842, a charged neutralizing device disposed proximate an electrospray discharge and along an evaporation region is used to provide the function of reducing an electrical charge of the droplets as the spray of droplets exits the electrospray device to prevent the droplets from disintegrating due to repulsive coulombic forces. For example, the electrospray device produces very highly charged aerosol particles which typically carry about 80%-95% of a Rayleigh limit of charge, on the order of 10-1,000 elementary units of charge.
As described in U.S. Pat. No. 5,247,842, a preferred neutralization process includes using a source of ionizing radiation (for example, radioactive polonium emitting alpha particles or a photon ionization source), or another source of ions, such as corona discharge. The source of ions is positioned proximate the electrospray discharge such that the droplets encounter the ions virtually immediately upon their formation. Additional sources of ions can be positioned further downstream along the evaporation region so that the droplets are further neutralized as they proceed downstream.
In such a device, the highly unipolarly charged particles, e.g., spray from the electrospray device, are exposed to the ions in the neutralizing device. However, more than 80% of the charged particles are lost within the neutralizer, e.g., to the walls, because of the high electric mobility of the particles. Further, such particles are lost due to the use of a high electric field needed for generating droplets encompassing the charged particles. The charged particles follow the high electric field from the point of generation to the walls and many are lost at the walls. Further, the space charge of the charged particles also cause the expansion of the stream of particles resulting in contact and loss to the walls. With such high charged particle loss, the amount of particles reaching the exit for provision to downstream devices, such as detection and characterization devices, is undesirably low.